A First Course in String Theory

14.3 Massive level in the open superstring.

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**What is a zero mode?**

p.315 “Being zero modes, these creation operators do not contribute to the mass-squared of the states.”

**How come there are creation operators that do not contribute to the mass-squared of the states?**

Before each creation operator, there is a multiple which is the same as the absolute value of the index of the creation operators.

If an index can be zero, the corresponding term can be zero.

— Me@2015.09.26 08:44 PM

Consider the NS-sector:

Equation (14.37):

For the NS sector, the values in are half-integers, thus cannot be zero. So every creation operator contributes to the mass-squared.

(p.312 “… the negatively moded coefficients , , , …, are creation operators, …”)

Consider the R-sector:

Equation (14.53):

For the R sector, the values in are integers, thus can be zero. So some of the creation operators are zero modes.

p.315 “… the eight fermionic zero modes …”

— Me@2015-10-11 11:01:44 AM

2015.10.11 Sunday (c) All rights reserved by ACHK